The preparation and use of organic isocyanates and ureas are closely parrelled by the analogous organo isothiocyanates and thioureas.
One of the most commonly and easily effected preparative routes to organo isothiocyanates is the thiophosgenation route where a salt of a primary amine (for example, in a slurry with toluene as the slurry agent) is reacted with thiophosgene. See, for example, U. K. Patent No. 779,806.
In German Patent No. 1,148,540, a metathesis reaction is carried out by treating substituted benzyl halides having at least one hydrogen on the halide carbon with potassium thiocyanate. An iodide or bromide catalyst is used to promote the reaction which is carried out in an inert solvent such as dimethylsulfoxide.
In a process somewhat similar to the above described thiophosgenation reaction, U.S. Pat. No. 3,530,161 discloses the reaction of an aromatic nitro compound with carbon monoxide and carbon disulfide in the presence of a catalytic amount of iron (II) or cobalt (IV) carbonyl. The preparation of aromatic isothiocyanates from such reaction is particularly facilitated by higher than ambient pressure.
Carbon disulfide has also proven useful in preparing various aryl and alkyl isothiocyanates by allowing it to react with a halomagnesium alkyl or arylamine (Gregnard reagent) forming, in situ a halomagnesium N-aryl(or alkyl)-N-halomagnesiodithiocarbonate. Thermal decomposition of the dithiocarbonate produces the desired aryl (or alkyl) isothiocyanate. See Sakai, et al, Bul. Chem. Soc. Japan, 48(10) 291 (1975).
While some of the processes of the above prior art yield acceptable quantities of the isothiocyanate product, they are generally quite inconvenient, utilizing reagents that are toxic (such as heavy metals and/or phosgeneous derivatives) or require difficult reaction conditions.
It has now been discovered that certain aryl isothiocyanates can be readily prepared in good yields by a simple thermal treatment. The isothiocyanates prepared by the process of the present invention are those of the formula ##STR3## where R, R.sub.1, and R.sub.2 are the same or different and are hydrogen or C.sub.1 to C.sub.6 linear or branched alkyl.
In the above compounds, R is preferably C.sub.1 to C.sub.6 linear or branched alkyl different from R.sub.1 and R.sub.2. Most preferably R.sub.1 and R.sub.2 are the same and are C.sub.1 to C.sub.6 linear or branched alkyl. Particularly preferred are compounds of the above formula where R is methyl, and R.sub.1 and R.sub.2 are the same and are ethyl.
The process of this invention is typically carried out by using as reactants diamine compounds of the formula ##STR4## and thiourea. The compounds are mixed without a solvent and heated to about a range of 150.degree. to 250.degree. C. It should be noted that inert, organic solvents may optionally be used. However, reaction times are usually longer due to dilution effect on the reacting molecules. Preferably, the reaction is carried out at temperatures of from about 175.degree. to 225.degree. C. for 2 to 30 hours, most preferably about 185.degree. to 215.degree. C. for about 3 to about 7 hours.
The isothiocyanates formed are conveniently separated from the reaction mass by vacuum distillation. However, any other conventional method is also acceptable, e.g., extraction, recrystallization, etc.
The isothiocyanate resulting from this reaction is, surprisingly, a monoisocyanate. Thus, illustrative compounds prepared by the process of the present invention include:
3,5-dimethyl-4-aminotoluyl-2-isothiocyanate; PA0 3,5-diethyl-4-aminotoluyl-2-isothiocyanate; PA0 3,5-diisopropyl-4-aminotoluyl 2-isothiocyanate; PA0 5,6-dimethyl-4-aminotoluyl-2-isothiocyanate; PA0 5,6-diethyl-4-aminotoluyl-2-isothiocyanate; PA0 5,6-dipropyl-4-aminotoluyl-2-isothiocyanate; PA0 2,6-dimethyl-4-ethyl-3-isothiocyanatoaniline; PA0 2,6-diethyl-4-ethyl-3-isothiocyanatoaniline; PA0 2,6-triisopropyl-3-isothiocyanatoaniline; PA0 2,6-dimethyl-4-propyl-3-isocyanatoaniline; PA0 2,6-diethyl-4-propyl-3-isocyanatoaniline; PA0 2,4,6-tripropyl-3-isocyanatoaniline; PA0 3,5-dimethyl-6-aminotoluyl-2-isothiocyanate; PA0 3,5-diethyl-6-aminotoluyl-2-isothiocyanate; PA0 3,5-dipropyl-6-aminotoluyl-2-isothiocyanate; PA0 3,5-dimethyl-2-aminotoluyl-4-isothiocyanate; PA0 3,5-diethyl-2-aminotoluyl-4-isothiocyanate; PA0 3,5-dipropyl-2-aminotoluyl-4-isothiocyanate; PA0 2,4-dimethyl-6-ethyl-3-isothiocyanatoaniline; PA0 2,4-diethyl-6-ethyl-3-isothiocyanatoaniline; PA0 2,4-tripropyl-3-isothiocyanataniline; PA0 2,4-dimethyl-6-isopropyl-3-isocyanatoaniline; and PA0 2,4-diethyl-6-isopropyl-3-isocyanatoaniline. PA0 bis(3,5-dimethyl-4-aminotoluyl-2)-thiourea; PA0 bis(3,5-diethyl-4-aminotoluyl-2)-thiourea; PA0 bis(3,5-dipropyl-4-aminotoluyl-2)-thiourea; PA0 bis(3,5-dimethyl-2-aminotoluyl-4)-thiourea; PA0 bis(3,5-diethyl-2-aminotoluyl-4)-thiourea; PA0 bis(3,5-diisopropyl-2-aminotoluyl-4),-thiourea; PA0 N-(3,5-dimethyl-4-aminotoluyl-2)-N'-(3,5-dimethyl-2-aminotoluyl-4)-thiourea ; PA0 N-(3,5-diethyl-4-aminotoluyl-2)-N'-(3,5-diethyl-2-aminotoluyl-4)-thiourea; and PA0 N-(3,5-diisopropyl-4-aminotoluyl-2)-N'-(3,5-diisopropyl-2-aminotoluyl-4)-th iourea.
After removal of the isothiocyanate by any of the conventional methods discussed above, a residue remains that produces a compound of the formula ##STR5## where R, R.sub.1, and R.sub.2 are as previously described.
These thioureas are high melting compounds that are sufficiently reactive to be used as heterogeneous chain extending agents in, for example, polyurethane, or polyimide, or polyamide preparations.
In the above thioureas, R, R.sub.1, and R.sub.2 are the same or different and are individually hydrogen or C.sub.1 or C.sub.6 linear or branched alkyl. Preferably, R is C.sub.1 to C.sub.6 linear or branched alkyl, different from R .sub.1 and R.sub.2. Most preferably, R.sub.1 and R.sub.2 are C.sub.1 to C.sub.6 linear or branched alkyl. Particularly preferred are R as methyl and R.sub.1 and R.sub.2 as ethyl.
Illustrative compounds of the present invention that are thioureas are as follows:
The process and compounds of the invention are illustrative by the following examples which are intended to not limit the scope of the invention in any way.